Download Reproduction - Net Start Class

Reproduction (7.14B)
Student Expectation
The student is expected to compare the results of uniform or diverse offspring from
sexual reproduction or asexual reproduction.
Key Concepts
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Key Concept 1: In asexual reproduction of prokaryotic cells, DNA is replicated
from the parent resulting in uniform offspring. These cells divide by binary
fission. Organisms composed of eukaryotic cells can also reproduce asexually by
forming spores, by budding, or by vegetative propagation.
Key Concept 2: In sexual reproduction of eukaryotic organisms, DNA is
combined and unique combination of dominant and recessive traits from two
parents create diverse offspring.
Teacher Background
Objective(s):
7.14B Recognize that inherited traits of individuals are governed in the genetic material
found in the genes within chromosomes in the nucleus.
Foundation:
Students have previously been introduced to the basic concepts of heredity by examining
and being aware of observable traits, such as eye color in humans or shapes of leaves in
plants. Such shared characteristics are different from learned behaviors, such as table
manners or learning a language. Students have likely also explored the basic concept of a
cell and that it contains a nucleus. They may even be aware that each human cell has 46
chromosomes, with all of a person’s DNA organized into two sets of 23 chromosomes.
During this grade level, students will begin to get more in-depth in their understanding
that constructs called chromosomes contain the DNA for these traits and that traits, such
as eye color, are passed from one generation to the next by each parent contributing a set
of chromosomes to an offspring. This is why children look similar to their parents.
Furthermore, which set of chromosomes gets inherited from each parent is random. This
is why siblings born from separate pregnancies look similar but not identical, and why
identical twins are just that, because they actually do both carry the same inherited sets of
chromosomes. Essentially, the DNA provides the instructions or recipe for “building” an
offspring, using the blueprint provided by the combination of the two individual parents.
Heredity is not merely observed within single species, however. Mapping the human
genome, as well as that of other species, has provided insight into how different species
are related to each other. Not only have mammals inherited traits such as mammary
glands and hair from a common ancestor, for example, but also about 75% of known
human disease genes have a recognizable match in the genome of fruit flies. This infers
that humans and fruit flies also share some common ancestry.
Students will also learn the difference between genotype and phenotype. A genotype is
the genetic makeup of an organism, while the phenotype is a description of how that
genotype is expressed in the organism’s morphology and physiology. Furthermore, a
genotype for a trait often includes two variations that are referred to as a dominant allele
and a recessive allele. When both a dominant allele and a recessive allele are present for a
trait, the dominant allele will mask the recessive allele’s expression of the trait. Only
when two copies of the recessive allele are present – one from each parent – is the
recessive form expressed. This concept is especially easy to understand when examining
phenotypic traits that are controlled by single genes. The ability to roll your tongue or the
presence of a Widow’s Peak hairline are examples of dominant expression of traits that
scientists believe are controlled by a single gene. If a person’s DNA that controls hairline
shape contains both the dominant allele (Widow’s peak hairline) and the recessive allele
(straight hairline), or a heterozygous state, then the person’s phenotype will show a
Widow’s Peak hairline. Individuals who have two recessive alleles, or a recessive
homozygous state, for the trait will have a straight hairline.
However, not all traits are controlled by single genes. Most inherited traits are controlled
by a combination of multiple genes. This fact makes genetic research especially
complicated when trying to figure out how defects and risks for disease are configured
into a person’s genotype. Surveying generational data, ongoing work on mapping
genomes, and other studies continue to further our understanding of how heredity works
and how medical professionals can predict, and possibly curb, health risks.
Wild animal and plant populations, of course, also demonstrate how traits are inherited.
There are eight genetic lineages of felines, for example. Lions, leopards, panthers,
servals, cheetahs, pumas, and mountain lions, all share genetic traits inherited from a
common ancestor. Genetic similarities can easily be observed between cat species,
including teeth, nose, hair, feet, and tail characteristics. Wild populations can suffer,
however, when their numbers are reduced. Inbreeding can occur, which results in low
genetic variation and often causes what are typically recessive, deleterious traits to show
up in the phenotypes in successive generations of offspring. Such growing homozygosity
in recessive traits is observable in the Florida Panther, for example. Abnormal phenotypic
traits include kinked tails and severe birth defects.
The principles of inheritance are also studied and applied in domestication of wild
species. Artificial selection, or selective breeding, has produced a variety of livestock
breeds and plant types that boost human population survival and growth, while pet breeds
provide comfort and companionship. For hundreds of generations, humans have bred
together two individuals with desirable traits in order to enhance those desirable traits in
their offspring. Unfortunately, also due to the principles of inheritance and the nature of
chromosomes and their contained DNA, not all of the desirable traits can be teased from
non-desirable traits. A hybrid plant that may produce a high seed yield may also have a
higher vulnerability to disease, for example. Desirable traits in a dog breed may also be
accompanied by a higher risk of hip dysplasia.
The educational groundwork done at this grade level is crucial for understanding the
more complex concepts in higher grade levels. More importantly, perhaps, is that these
basic concepts of heredity are applicable to multiple real-world fields and applications,
such as in our everyday casual observations and conversations, medicine, wildlife
conservation, and food supplies.